All conventional linear amplifiers are subject to a reduction in the amplification efficiency when a partial modulation control is effective. This fact is independent of the amplifier operating point. In the present context the term "conventional amplifiers" means amplifiers in which the main supply voltage or the load are not dynamically controlled as a function of the instantaneous output power. Considerations generally applicable to such amplifiers show that the amplification efficiency cannot be better than being proportional to the signal voltage. Since practically all amplifiers have this characteristic, it is conventional to specify the amplifier characteristics at full input (output) voltage. The reduction in the amplification efficiency at partial output voltage or power is accepted as an unavoidable fact of nature.
Linear amplifiers of satellite transponders must be so controllable that relatively infrequent peak values are still transmitted without distortion. Such relatively infrequent peak values are formed by the vectorial addition of individual peak values which in turn are formed by the vectorial addition of the individual user signals (SCPC: single channel per carrier) in the transponder. As a result, the average signal power or voltage of the transponder becomes 4 to 8 dB smaller as compared to a full output power or voltage. This phenomenon is known as "backoff" and such backoff has the disadvantage that the efficiency of the transponder system becomes 1.5 to 3 times smaller than the characteristic values would tend to indicate for a full control or modulation.
The reasons for the reduction in the amplification efficiency are the following. Linear amplifiers are operated by applying a constant supply voltage to the collector. The amplifier is so dimensioned that on full output power or voltage the output transistor or the electron vacuum tube is optimally matched to the load. When a partial output power or voltage occurs, for example, when the output voltage is only 50%, the output power is reduced to 25% of the original output power due to the relationship (P=U.sup.2 /R), wherein P is the output power, U is the output voltage, and R is the load. Since the operating voltage of the amplifier is constant, the d.c. current input power is reduced only to 50% at this so-called half current operation. Thus, the problem is that the matching of the amplifier at half output voltage is actually mismatched. In fact, the supply voltage should also be reduced to one half in order to maintain a good amplification efficiency.
The same problem is encountered in amplitude modulated radio transmitters. Thus, any methods developed in connection with radio transmitters with regard to the just mentioned problem, are also basically usable in connection with satellite transponders. However, so far only AMSAT has used such methods in space.
Basically, there are two ways of keeping the amplification efficiency constant. One method involves modulating or controlling the collector voltage of the transmitter transistor as a function of the input signal. The other method involves using two amplifier channels which work into suitable output networks. In both instances, the load impedance at the amplifier output becomes variable so that for all loads the optimal matching is maintained.
Both conventional methods have been used in connection with radio technology and with corresponding modifications in amateur satellites. The following literature is relevant in this connection.
(1) A new high Efficiency Power Amplifier for Modulated Waves, W. H. Doherty, Proc. IRE, Vol. 24, Sept. 1936, pp 1163. PA1 (2) High Power Outphasing Modulation, H. Chireix, Proc. IRE, Vol. 23, Nov. 1935, pp. 1370. PA1 (3) Instruction Manual of the VP-100 Transmitter, Gates Radio Corp., Quincy, Illinois, USA. PA1 (4) Lineare Nachrichtentransponder durch nichtlineare Signalzer legung, K. Meinzer, Inaugural-Dissertation, Marburg, Federal Republic of Germany, 1973. (Linear Communication Transponders by Non-linear Signal Separation).
Special problems occur, however, in connection with L-band transponders. In the L-band, especially in the range of 1500 to 1700 MHz, the output powers presently obtainable with semiconductor amplifiers are within the range of 20 to 40 W (PEP-Peak Envelope Power). However, in most transponder applications larger output powers are required so that a parallel connection of a substantial number of amplifier modules is unavoidable. The large number of amplifiers causes a problem because hybrid matching circuits are required and these hybrid matching circuits make it impossible to take advantage of the principle of a variable amplifier load output impedance. A variation of the collector voltage, for instance by means of a pulse width modulator, regularly fails because the amplifier band-width cannot exceed a few MHz. This is so because the modulator must be capable of handling three times the transponder band-widths.